The Urochordata , also known as Tunicates and Ascidians , are more commonly known as "sea squirts." They belong to the same phylum as the vertebrates although adults do not have a backbone. During development the free-swimming larvae possess a tail, a dorsal nerve cord, a dorsal stiffening structure (not composed of bone) called the notochord, and gill slits in the throat (pharynx) region.

Tunicates are among the most common marine invertebrates with around 3,000 species. Most tunicates live attached to a hard surface on the ocean floor and are commonly known as sea squirts (or cunjevois) and sea pork. They are found at all depths of the ocean. Other tunicates – such as salps, doliolids and pyrosomes – live in the pelagic zone as adults and are free-swimming or drifters..

The name tunicates comes from the tunic surrounding their body. The name sea squirt comes from the way water is pushed out through the exhalant siphon.

Red sea squirt

The inhalant siphon is used to take in food and water, and the exhalant siphon expels waste and water.

The tunic is a thick, usually opaque, covering protecting their barrel-shaped bodies from predators. It is made from a material very similar to cellulose. On the inner surface of the tunic is a thin epidermis, it is this which secretes the tunic. On the inside of the epidermis is a thicker dermis (body wall) and bands of muscles which can squeeze the tunic forcing a jet of water from the exhalant siphon.

Most of the space within the tunicate body is taken up by the atrium (a large cavity). This contains the enlarged pharynx, which has large numbers of small pores or slits in its walls through which water can pass. It is connected to the digestive system at one end and to the inhalant siphon at the other, this is effectively the animal's mouth.

The tunicate's pharynx is covered by tiny hairs (ciliate cells) which allow the consumed food to pass down through to the oesophagus. The digestive system is U-shaped, the anus emptying directly to the outside.

Tunicates are filter feeders, feeding by drawing often hundreds of litres of water each day through the inhalant siphon. This water passes through the pharynx where small particles are filtered out before the water is expelled through the exhalent siphon. The water current is caused by beating cilia. Water can also be pushed out of the atrial cavity by muscular contractions of the tunic if the tunicate is threatened. The small particles of plankton, etc, are trapped on a continually moving layer of mucous. This mucous is released by special cells and is moved across the surface of the pharynx by the beating of many small cilia, until it is passes into the digestive system where the food particles and mucous are digested.

Body structure of a sea squirt

Most tunicates are hermaphrodites. They avoid self fertilisation by either having the eggs and sperm reject each other, or by having the eggs and sperm mature at different times. Sperm are released into the sea but the eggs are retained within the body where they are fertilised by sperm brought in with incoming water. The eggs are brooded within the body until they hatch.

The larvae look like tadpoles and are far more obviously members of the phylum Chordata than the adults. Tunicate larvae do not feed but search for a suitable location on the sea floor and then settle in a head down position. They attach themselves to the sea floorusing special adhesive glands in the front of their head. Once settled they undergo metamorphosis during which they lose their tail and their ability to swim. The remainder of the body twists through 180 degrees in order to become a small tunicate. Most tunicates live about one year as adults.

They are eaten by sharks, skates, and other bottom-dwelling animals including periwinkles. Many have poisonsous flesh to deter predators.

Sea squirts have shown promise as sources of chemicals which may be used to treat cancers and other medical conditions.

Warty sea squirt
Warty sea squirt (Pyura haustor); see how this animal feeds
by creating water flow with its incurrent and excurrent siphons.

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